This invention relates to a varistor made of an oxide-semiconductor.
As an element for circuit to which a semiconductor is applied, there has been used a varistor (i.e., a resistor whose resistance varies non-linearly relative to the applied voltage). Typically, a varistor composed of a sintered ZnO to which various kinds of oxides are added, has been known to the art. This kind of varistor has non-linear volt-ampere characteristic, that is to say, its resistance decreases abruptly with the rise of the voltage so that the current increases remarkably. Therefore, such varistor has been practically used for the purpose of absorbing abnormal voltage and stabilizing voltage.
The performance of a varistor is generally evaluated by the volt-ampere characteristic represented approximately by the following equation: ##EQU1## wherein I is the current flowing in the varistor; V is an applied electromotive force (voltage); C is a constant; and .alpha. is a non-linearity coefficient.
Accordingly, the general performance of a varistor can be indicated by the two constants of C and .alpha., and usually is indicated by voltage V.sub.1 at 1 mA in place of C.
While the above-mentioned ZnO-system varistor has many advantages such that its volt-ampere characteristic can be controlled optionally, it has such a drawback in cases where it is used for providing a pulse whose rise time is short. That is, the conventional ZnO-system varistor has been disadvantageous in that the absorbability of an overvoltage in a pulse of a short rise time is so extremely lowered that it can not perform a function which has been of great account in a varistor. Such a phenomenon is considered to occur for the following reasons:
In general, when an overvoltage is applied, the varistor absorbs the overvoltage by conducting a current corresponding to the voltage. However, the response current (the pulse response) which has resulted by the application of a stepwise voltage to a conventional ZnO-system varistor changed characteristically with the time lapse. More specifically, the charging current which varies depending upon the capacitance of the ZnO-system varistor flows at first, and then the current, after reaching a peak, decreases exponentially relative to the lapse of time, and thereafter the current inherent to the ZnO-system varistor increases gradually at a time constant of from several to several tens of microseconds to converge on the current value as indicated by the afore-mentioned equation of the volt-ampere characteristic.
In other words, the current of the conventional ZnO-system varistor is extremely limited over a time range of several microseconds immediately after application of a voltage. With respect to the overvoltage pulse of a short rise time, the sufficient current does not flow in such a varistor during the time range mentioned above, whereby the overvoltage-absorbability is extremely lowered.
Recently, attempts have been made to improve the pulse response as disclosed in Japanese Laid-Open patent application Nos. 61789/1977 and 22123/1981. However, none of them show pulse response and non-linearity sufficient for practical use.